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Wednesday, March 28, 2012

MCC Update: ATV 3 Docking

Today we are docking ATV 3. The Automated Transfer Vehicle (ATV) is a European Space Agency unmanned cargo carrier. They come up, full of food, clothes, water, science payloads, and more. The crew empties them of all these good things, fills them with trash, and sends them off. Since they are unmanned, they do not need to safely return. Instead they burn up in the atmosphere on re-entry... a giant, space-age incinerator. Two have previously visited space station, hence the number 3.

Dockings of any kind are a busy day for the ISS Mission Control Team. They are considered complex operations and carry a higher level of risk for the station and crew than an average day, running circles around the Earth. In orbit, spacecraft are going extremely fast (ISS goes about 17,000 miles per hour), so you have two fast moving objects coming together in space. Of course, the relative speed (difference between one and the other) should be very small, but that doesn't mean something couldn't go wrong. To be sure that everything is as safe as possible, we do a lot of advance preparations.

At the Spartan console, we have 12 rotating joints: 2 TRRJs in blue (Thermal Radiator Rotary Joint- which move the external cooling radiators to get the best heat rejection angle), 2 SARJs in pink (Solar Alpha Rotary Joint- which move 4 solar arrays each in one dimension) and 8 BGAs in yellow (Beta Gimbal Assembly- which each move a single solar array along a different axis from the SARJ). Between the SARJ and BGA we can point the solar arrays pretty dead on at the sun to maximize power generations.

Source- follow this link for more truss and component details. And an interactive video.

Well, usually we can point them at the sun. However, when a visiting vehicle like ATV is coming in, we have to park them at specific angles.This is for a couple reasons. We need them to be positioned where the thrusters from either the ISS attitude control system or the ATV attitude control system will not damage the arrays. Thrusters can be powerful and we have concerns both of them breaking the array structure, which could be dangerous to the crew if a broken solar array bit hits a module. We also need to protect them from being eroded by the thruster blast, diminishing their power generation capabilities. We also have concerns called "multipathing"- the communication system of the visiting vehicle works a lot like an aircraft's radar to find the range to the ISS as it is approaching and steer the unmanned craft in. The arrays and radiators are shiny and reflective, which could cause the signal from the vehicle to bounce off and cause the ATV to have no idea where it really is in space. Bad news for docking! Since we position the arrays in what is probably not the best place for power generation, we also have to be concerned with power limitations- some equipment on-board will need to be powered down to ensure we stay within our power generation abilities.
Starting at 10am today (Wednesday), I'll be in mission control, helping to prepare the ISS for ATV 3 docking. You can also watch the docking live on NASA-TV starting at 3:45 CDT or at http://www.nasa.gov/multimedia/nasatv/schedule.html.